1. Field of the Invention
The present invention relates to communications; more specifically, wireless communications.
2. Description of the Related Art
In wireless communication systems such as Code Division Multiple Access (CDMA) systems the number of users that are supportable by a base station is limited by resources such as available transmit power and a number of orthogonal codes that are used to distinguish channels. In many CDMA systems the orthogonal codes are derived from a set of 16 orthogonal Walsh codes.
In CDMA systems that support both voice and data communications the power and code resources are shared between the voice and data users. Typically, each voice user is provided with one orthogonal code and sufficient transmit power to maintain an acceptable signal to noise ratio or frame error rate. The remaining transmit power and orthogonal codes are provided to one data user at a time on a time shared basis. As a result, for a short period of time, each data user receives all the remaining transmit power and orthogonal codes that are available after satisfying the requirements of the voice users.
Typically, a data rate provided to a data user is picked from a relatively large set of standardized data rates. The data rate provided to the data user is based on a signal quality measurement such as a carrier to interference ratio measured by the user's terminal. Since a relatively large set of standard rates are available, a relatively large number of overhead bits are required for the user's terminal to specify which of the standardized rates are to be used based on the carrier to interference ratio measured by the mobile unit. Using the large number of bits to specify the data rate is wasteful of limited signaling channel bandwidth.
In addition, older CDMA systems use carrier to interference ratios measured at a data user's mobile terminal to select base stations during handoff periods. Using carrier to interference ratios for base station selection during handoffs involving a data user may result in the user selecting a base station that does not provide the maximum overall data rate to that user.
In the data only evolution of the third generation CDMA standard (hereinafter referred to as 3G-1x EVDO), voice and data services are provided using separate frequency carriers. That is, the voice and data signals are transmitted over separate forward links defined by different frequency carriers. Data is transmitted over a time division multiplexed carrier at fixed data transmit powers but at variable data rates. To improve system throughput, the system allows the wireless unit with the best channel, and thereby the highest rate, to transmit ahead of wireless units with comparatively low channel quality. 3G-1x EVDO uses a fast rate adaptation mechanism whereby the wireless unit performs the rate calculation at every slot using measurements of a pilot signal broadcast from the base station and reports back the rate at which it is going to receive data from the base station at every slot.
The measured signal to interference ratio (SIR) at the receiver is used to determine a data rate which can be supported by the receiver. Typically, the determined data rate corresponds to a maximum data rate at which a minimum level of quality of service can be achieved at the access terminal. Higher measured SIR translates into higher data rates, wherein higher data rates involve higher order modulation than lower data rates.
The simplified Forward Traffic Channel structure in a 3G-1x EVDO system is shown in FIG. 1. The sequences of modulation symbols after modulation repetition/puncturing is demultiplexed to form 16 pairs (in-phase and quadrature) of parallel streams. Each of the parallel streams is covered with a distinct 16-ary Walsh function at a chip rate to yield Walsh symbols at 76.8 ksps. The Walsh-coded symbols of all the streams are summed together to form a single in-phase stream and a single quadrature stream at chip rates of 1.2288 Mcps.
In a 3G-1x EVDO, the modulation, coding and the number of Walsh codes is fixed for a given data rate and is known by the access terminal and the base station. The data rate predictor in a 3G-1x EVDO system takes measured SIR, modulation and coding parameters for all the rates, and the frame error rate (FER) target as inputs and outputs the supportable data rate (see FIG. 2).
Another evolution of the third generation CDMA standard called 1xEV-DV support circuit-switched voice and data as well as packet-switched high-speed data on the same 1.25 MHz spectrum. The support of packet-switched high speed data users is provided by means of a new, shared channel that serves one packet data user at a time in a time-multiplexed manner (similar to 1xEV-DO). The Walsh codes are dynamically shared between circuit-switched services and packet services. Therefore, the number of codes available for data change dynamically due to circuit switched call arrivals and call departures.
The shared packet data channel is defined based on the use of a multitude of fixed spreading factor codes e.g. spreading factor SF=16. The number of codes of SF=16 that are available for the shared packet data channel would vary depending on the codes being used by the dedicated channel users. The same is true of the base station transmitter power available for the shared channel. The values of available transmit power as a fraction of the total and the available code space are broadcast on a newly defined channel enabling the UE to make a better estimate of the supportable rate (or C/I).
The simplified Forward Traffic Channel structure in a 3G-1x EVDV system is shown in FIG. 3. Each one of the parallel streams of the forward channel is covered with a distinct 16-ary/32-ary/64-ary/128-ary Walsh function at a chip rate to yield Walsh symbols at 76.8 ksps/38.4 ksps/19.2 ksps/9.6 ksps. The Walsh-coded symbols of all the streams are summed together to form a single in-phase stream and a single quadrature stream at chip rates of 1.2288 Mcps.